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OOP – Private Attributes (underscore convention)

Prefixing an attribute with _ signals it is private and should not be accessed directly from outside the class

What it does: A single underscore is a convention meaning 'do not access directly'. Getters provide controlled read access to private attributes. _name signals 'treat this as private' — Python won't stop you accessing it though. __name becomes _ClassName__name — Python mangles it to make accidental access harder. A setter can check that a new value is valid before storing it. A getter method reads a private attribute safely without exposing it directly. Hiding attributes forces callers to use your methods, where you can validate changes. Python allows it technically — the underscore is just a warning not to do it.

Common mistakes: Directly setting _balance bypasses validation. Use deposit() method. __balance is name-mangled. Access through a getter method.

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Learning

Duo

What does a single underscore _ prefix mean?

Convention: internal use, don't access directlyThe variable is deleted

Matches

❌ obj.__balance

↓

Error: Double underscore triggers name mangling

Applying

Meanings

Term

_variable

Meaning

Convention for internal use - don't access directly from outside the class.

Practice

class BankAccount:

Testing

Gaps

def (self, bal):

Check Answer

Spots

1acc = BankAccount(100)

2acc._balance = -500

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Quick Questions

1. What does a single underscore _ prefix mean?

A. Convention: internal use, don't access directly ✓

B. The variable is deleted

_name signals 'treat this as private' — Python won't stop you accessing it though

2. Does Python enforce single-underscore privacy?

A. Yes you can't access it

B. No it's just a convention ✓

Single underscore is a naming convention only — nothing prevents external access

3. What does double underscore __ do?

A. Name mangling makes it harder to access ✓

B. Deletes the variable

__name becomes _ClassName__name — Python mangles it to make accidental access harder

4. Why use getters and setters?

A. They're faster

B. To control and validate access to attributes ✓

A setter can check that a new value is valid before storing it

5. def get_balance(self): what is this?

A. A getter method that returns the balance ✓

B. A constructor

A getter method reads a private attribute safely without exposing it directly

Match the Pairs

obj.__balance → Error: Double underscore triggers name mangling

def getBalance(self): → Error: Use snake_case: get_balance

self._balance → Valid: Single underscore signals internal use

def get_balance(self): → Ok: Getter method to access private data

10 // 3 → Output: 3

10 % 3 → Output: 1

Key Term

_variable

Convention for internal use - don't access directly from outside the class.~~__variable

Fill in the Blanks

1. class Account: def __init__(self, bal): self._balance = bal

2. class Account: def get_balance(self): return self._balance

3. class Account: def deposit(self, amount): if amount > 0: self._balance += amount

4. class Secret: def __init__(self, val): self.__val = val

5. acc = Account(100) print(acc.get_balance( ))

Spot the Error

Question 1

A acc = BankAccount(100)

B acc._balance = -500

C print(acc.get_balance())

Line B — Directly setting _balance bypasses validation. Use deposit() method.

Question 2

A class Account:

B def __init__(self, bal):

C self.__balance = bal

E acc = Account(100)

F print(acc.__balance)

Line F — __balance is name-mangled. Access through a getter method.

Question 3

A class Account:

B def deposit(self, amount):

C self._balance += amount

Line C — No validation should reject negative deposits

Question 4

A class Person:

B def __init__(self, age):

C self._age = age

D def set_age(self, age):

E self._age = age

Line E — Setter has no validation should check age > 0

Question 5

A class Account:

B def __init__(self, bal):

C self._balance = bal

D def get_balance():

E return self._balance

Line D — Missing self parameter in get_balance method

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